The disclosed subject matter relates generally to telecommunications and, more particularly, to a method and apparatus for shared positive protection.
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. A line card typically includes at least one subscriber line interface circuit (SLIC) as well as a subscriber line audio-processing circuit (SLAC). The functions of the line card include range from supplying talk battery to performing wake-up sequences of circuits to allow communications to take place.
Subscriber line interface circuits (SLICs) have been developed to provide an interface between a low voltage signal path in a telephone central office and a high-voltage telephone subscriber line. The SLIC provides functions such as off hook detection, ringing signal generation, and battery feed to the subscriber line. The subscriber line consists of a telephone transmission line, including two conductors referred to as A and B or tip and ring, and the subscriber telephone equipment coupled across the tip and ring conductors (i.e., the load). The subscriber line and the subscriber telephone equipment are also referred to as a subscriber loop.
The SLIC provides power from the telephone central office to the subscriber line in response to a received battery voltage. The battery voltage is a DC voltage supplied to the SLIC to power the SLIC and the subscriber line. The SLIC supplies a DC current at the battery voltage to the subscriber line. Superimposed on the DC current are AC signals of audio frequency by which information is conveyed between the subscriber and the central office. The battery voltage is generated at the central office, either by a depletable energy storage device such as a battery or by a DC generator, for supply to the SLIC. In a central office, one battery or DC generator supplies the battery voltage to many SLICs and their associated subscriber loops.
To meet safety requirements it is common to use a positive battery, VBP, along with a negative battery, VBH, to generate sufficient voltage for ringing generation. To save power and reduce power dissipation on the SLIC for short loop applications, a third supply is often used for DC feed along with VBH. The third supply is typically lower in amplitude than VBH, and is often referred to as VBL.
To protect the SLIC from voltage excursions, such as lightning surges or power cross events, a protection scheme is employed to connect the tip or ring line experiencing the event to ground. Typically one protector is employed to track the VBP supply and another protector that tracks the VBH supply.
FIG. 1 is a circuit diagram illustrating a conventional protection circuit 100. In FIG. 1, multiple interface lines 105 for interfacing with external subscriber lines are shown (i.e., designated as TIP1-n and RING1-n). Each interface line 105 has protection resistors 110, a negative overvoltage protector 120 referenced to VBH, and a positive overvoltage protector 130 referenced to VBP. Exemplary overvoltage protectors are available commercially, such as a TISP8200M device for the negative overvoltage protector 120 and a TISP8201M for the positive overvoltage protector 130, both offered by Bourns, Inc. of Riverside, Calif. Gate capacitors 125, 135 are provided at the gate inputs of the protectors 120, 130 for maintaining the reference voltage for the protectors 120, 130 during transients.
The negative overvoltage protector 120 initially clips overvoltages close to the VBH value. If sufficient current is available from the overvoltage, then the negative overvoltage protector 120 will crowbar into a low voltage ground referenced on-state condition. As the overvoltage subsides, the high holding current of the negative overvoltage protector 120 prevents DC latchup with the SLIC output current. Similarly, the positive overvoltage protector 130 initially clips positive overvoltages close to the VBP value. If sufficient current is available from the overvoltage, then the positive overvoltage protector 130 will crowbar into a low voltage ground referenced on-state condition. As the overvoltage subsides the SLIC pulls the conductor voltage down to its normal negative value and this commutates the positive overvoltage protector 130 into a reverse biased condition. During an overvoltage transient, increased current may pass through the protection resistors 110, which are positive coefficient devices, causing them to tri-state and thereby isolate the associated interface line 105.
The dual protector protection scheme of FIG. 1 is costly, since each interface line supported by a line card requires two protectors 120, 130 and two relatively expensive gate capacitors 125, 135.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.